Methyl anion inversion barrier

Furthermore, just as the pyramidal structure in amines is inverting rapidly, so it would be expected that free carbanions will also invert their stereochemical configuration rapidly (III). Theoretical calculations have yielded7 a value of about 11 kcal.mole-1 for the barrier to inversion of the methyl anion, and it may be expected that a carbanion formed free in solution will, on further reaction, yield products derived equally from the two possible configurations. Should the... 

The cis-trans isomerization of 4-methyl-2-oxo-2/T-l,3,2-dioxaphos-phorinans (123) and (124) has been studied by n.m.r. spectroscopy on a sample of 90% /ra j -phosphorinan. The reaction is first-order, with an activation energy lower than the inversion barrier in tertiary phosphines. In view of this, a mechanism involving deprotonation and inversion of the phosphite anion is proposed. 

As in other areas of organic chemistry, computational investigations have provided important insights into the nature of carbanions, and the results complement the experimental studies. For example, ab initio calculations suggest that the inversion barrier of a methyl anion is ca. 2.2kcal/mol and that the inversion barrier of the ethyl anion is 3.3 kcal/mol. These values contrast with the <0.2 kcal/mol inversion barrier of the resonance-stabilized cyanomethyl anion on the one hand and the ca. 15 kcal/mol barrier for inversion of the cyclopropyl anion via a highly strained transition structure on the other hand. By comparison, the inversion barrier of ammonia is about 5.5 kcal/mol. ... 

Quantum chemical calculations allow an assessment of possible intermediates during the racemization process, since the results can be correlated with experimental observables. Starting from model system 4, it is possible to locate transition state TS-4 for the inversion at the silicon center (Fig. 1). The calculated barrier (159 kJ/mol for the inversion) is decreased drastically if the methyl groups at the silicon center are exchanged by phenyl groups, because these substituents can stabilize the transition state. These results prove once more the importance of the presence of solvated molecules in calculations in order to obtain the sufficient description of inversion processes and barriers, which can be compared with experimental results (inversion barrier for MesSi 199 kJ/mol). Nevertheless, when calculations are considered in the present literature, free silyl anions and unsolvated silyllithium compounds are still discussed as appropriate model systems [14]. 

The preceding experiments provide chemical evidence for significant configurational stability of silyl anions. An NMR study of diisopropylphenylsilyllithium showed that the isopropyl methyl groups are anisochronous up to 185°C, indicating that the barrier to inversion at silicon in the silyl anion is greater than 24 kcal/mol. (30). 

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